Regulating SnO2 surface by metal oxides possessing redox or acidic properties: The importance of active O2?/O22? and acid sites for toluene deep oxidation

Article abstract of DOI:10.1016/j.apcata.2020.117755

To explore the surface doping effects of metal oxides possessing redox (Cr, Co, Mn) or acidic (In, Bi) properties on SnO₂, SnO₂ doped by different metal oxides has been designed. All the doping metal oxides disperse finely on the SnO₂ support surface, which can improves the reaction activity, obeying the order of 1%Mn/SnO₂ > 1%Co/SnO₂ > 1%Cr/SnO₂ > 1%Bi/SnO₂ > 1%In/SnO₂ > SnO₂. By modifying SnO₂ surface with Cr, Co and Mn oxides, both the active surface oxygen (O₂^? and O₂^2?) and acid sites quantities have been significantly improved, thus resulting in remarkable activity increment. Whereas, by modifying SnO₂ surface with In and Bi oxides, only the surface acid sites amount has been improved, hence having limited activity improvement effect. 1%Mn/SnO₂ catalyst possesses considerable amount of both types of active sites, thus showing the optimal toluene deep oxidation activity in all the catalysts.

Full text of DOI:10.1016/j.apcata.2020.117755

Journal Pre-proof
Regulating SnO surface by metal oxides possessing redox or acidic
2
−
2−
properties: The importance of active O2 /O2 and acid sites for toluene
deep oxidation
Yao Guo, Lanling Zeng, Xianglan Xu, Yaqian Liu, Yang Liu,
Xiongcong Fu, Zezhou Gao, Zijian Qian, Junwei Xu, Xiuzhong
Fang, Xiang Wang
PII:
S0926-860X(20)30348-3
DOI:
https://doi.org/10.1016/j.apcata.2020.117755
APCATA 117755
Reference:
To appear in:
Applied Catalysis A, General
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Accepted Date:
7 June 2020
15 July 2020
20 July 2020
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Regulating SnO
2
Surface by Metal Oxides Possessing Redox or Acidic
Properties: The Importance of Active O -
/O
2-
and Acid Sites for Toluene
2
2
Deep Oxidation
a
a
a
a
a
a
a
a
Yao Guo , Lanling Zeng , Xianglan Xu , Yaqian Liu , Yang Liu , Xiongcong Fu , Zezhou Gao , Zijian Qian ,
a
a
a *
Junwei Xu , Xiuzhong Fang , Xiang Wang
a
Key Laboratory of Jiangxi Province for Environment and Energy Catalysis, College of Chemistry, Nanchang
University, Nanchang, Jiangxi 330031, China
*
Corresponding author. E-mail: xwang23@ncu.edu.cn (X. Wang)
1

Graphical abstract
Highlights





Doping SnO
2
surface with redox or acidic nature improves toluene oxidation activity
> 1%Co/SnO > 1%Cr/SnO > 1%Bi/SnO > 1%In/SnO
With the order of 1%Mn/SnO
2
2
2
2
2
> SnO
2
Surface active oxygen and acidic sites interaction controls the reaction activity
Redox Cr, Co, Mn oxide doping enriches both kind of surface active sites
Acidic In, Bi oxide doping enriches only surface acidic sites
2

Abstract
To explore the surface doping effects of metal oxides possessing redox (Cr, Co, Mn) or acidic (In, Bi) properties
on SnO
on the SnO
%Co/SnO
oxides, both the active surface oxygen (O
2
, SnO
2
doped by different metal oxides has been designed. All the doping metal oxides disperse finely
2
support surface, which can improves the reaction activity, obeying the order of 1%Mn/SnO >
2
1
2
> 1%Cr/SnO
2
> 1%Bi/SnO
2
> 1%In/SnO
2
> SnO
2
. By modifying SnO surface with Cr, Co and Mn
2
-
2-) and acid sites quantities have been significantly improved,
surface with In and Bi oxides, only
the surface acid sites amount has been improved, hence having limited activity improvement effect. 1%Mn/SnO
2
and O
2
thus resulting in remarkable activity increment. Whereas, by modifying SnO
2
2
catalyst possesses considerable amount of both types of active sites, thus showing the optimal toluene deep
oxidation activity in all the catalysts.
-
Keywords: SnO
2
-based catalysts, toluene oxidation, surface modification, surface acid sites, surface O
2
and
2
-
O
2
anions
3

1. Introduction
Volatile organic compounds (VOCs) from both stationary and mobile sources could cause the generation of
secondary pollutions, such as smog, organic aerosols and tropospheric ozone damages, and eventually harm the
environment and human health [1-2]. Therefore, it is of great significance to reduce VOCs emissions. Catalytic
oxidation is one the most feasible and economical ways to remove the pollution of VOCs [3-4]. Supported
precious metal catalysts like Pt, Pd, Au have been extensively studied in VOCs combustion because of their low
temperature activity, high performance and good stability [5-10]. However, the expensive price and scarce
resources have limited their widespread applications. Over recent years, researchers have paid more and more
attention to base metal oxide catalysts such as the CoO
oxides [18-19].
X
[11-12], CeO [13-14], Mn-Ce oxides [15-17], Cu-Mn
2
The surface oxygen abundance and reactivity of a metal oxide catalyst are very crucial for its VOCs oxidation
performance. Ye et al prepared 3D hierarchical Co
3
4
O nano-catalysts with various morphologies for toluene
oxidation, the big specific surface areas, high defect structures with richly absorbed oxygen species and Co3+
cations are responsible for the excellent catalytic performance [20]. Dai et al have prepared MnO
2
, Cr
2
O
3
, Co
3
4
O ,
NiO catalysts with different morphologies for toluene deep oxidation [21-24]. They found that samples
possessing high adsorbed oxygen concentrations and low-temperature reducibility contributed significantly to the
catalytic performance. Guo et al also reported that the abundant surface oxygen species and low-temperature
reducibility played critical roles for efficient VOCs deep oxidation over layered copper manganese oxide [25].
Moreover, Chen et al found that a 3Mn1Ce oxide catalyst exhibits better catalytic activity than the individual
oxides for toluene oxidation, which was ascribed to the existing of higher concentration of active lattice oxygen
on the surface [26].
On the other hand, the surface acid sites of a metal oxide catalyst are also significant for its VOCs oxidation
activity. Qi et al reported that for the combustion of chlorinated VOCs over Nb
2
O -based solid super acid
5
materials, stronger acidity can facilitate the rupture of the C-Cl bonds to form HCl and organic intermediates,
4

which will then be converted into CO
2
and H
2
O on the redox sites [27]. Li et al used HNO
3
solution to treat Co
3
O
4
and proved that surface acidity could play a key role in hydrocarbon oxidation, because the surface acid sites can
efficiently adsorb reactant molecules [28]. More recently, our group’s work has also demonstrated that surface
acidity is one of the predominant elements to decide the toluene deep oxidation activity for SnO
29-31]. Therefore, besides surface redox property, surface acidity of a metal oxide catalyst is also critical for
VOCs deep oxidation.
SnO possesses abundant active surface and lattice oxygen species as well as surface Lewis acid sites. In
2
-based catalysts
[
2
addition, it has both good physical and chemical stability, which is a good candidate as catalytic materials. Up to
date, it has been broadly investigated and used as gas sensors and electrodes [32-33]. In contrast, its catalytic
chemistry properties have been much less studied. Solymosi et al indicated that SnO
for chlorinated hydrocarbons combustion [34]. Eguchi et al reported that a synergistic interaction between Pd and
SnO was observed during adsorbing oxygen for methane combustion [35]. Jiang et al reported that a special Au
and SnO interaction affects strongly the reactivity of the oxygen species, the surface Sn oxidation states, and the
types and concentration of the adsorbed surface oxygen species for benzene and methanol oxidation [36]. Over
past 10 years, our group have systematically probed the structure-reactivity relationship of SnO -based catalytic
materials for various reactions. It was revealed clearly that its surface oxygen vacancies can induce the formation
2
promoted by Cr is reactive
2
2
2
-
2-
of both O
2
and O
2
anions, which is important for toluene oxidation [31]. Through adding CuO or alkali metal
oxides on its surface, the quantities of these reactive surface oxygen species can be remarkably enlarged, thus the
soot oxidation activity of the modified catalysts can be sremarkably enhanced [37-38]. In addition, by increasing
its surface area or modifying its surface or lattice matrix with a secondary metal oxide, its surface acidity is able
to be enhanced, which can facilitate the ability to adsorb and activate reactant molecules, such as toluene, NO
and NH , thus eventually improving its activity for toluene oxidation and NO reduction by NH [30,39].
In this work, with the target to unravel the surface doping effects of metal oxides with redox or acidic properties
on SnO surface, and engineering catalysts owning both abundant redox and acid sites for efficient toluene
oxidation, SnO
x
3
x
3
2
2
modified by different metal oxides have been prepared with a simple impregnation method. It
5

was found that even loading a small quantities of In, Bi, Cr, Co and Mn oxides on SnO surface can increase its
2
toluene oxidation activity evidently, among which Mn oxide exhibits the optimal promotion effects. By using
multiple means, the surface properties and active sites have been explored, which are correlated with the toluene
oxidation activity.
2
2
. Experimental
.1 Catalyst preparations
The chemicals here are of A.R. grade. The catalysts were fabricated by an impregnation method by using
aqueous nitrate solutions as the precursors. For example, a certain amount of Co(NO
3
)
2
6H
2
O (A.R) was
-1
dissolved into distilled deionized (DDI) water to prepared first an aqueous solution (0.5 mol L ). Then suitable
amount of CVD-SnO
2
supports was added into the solution under constant stirring for 2 h at the room temperature.
o
Afterwards, the mixture was stirred further in a water bath at 80 C to evaporate the water. Lastly, the mixture
o
o
o
-1
was dried at 110 C overnight and calcined at 450 C with a ramping rate of 2 C min for 4 h in air. The final
catalyst was named 1%Co/SnO . All the other catalysts were synthesized with the same method and having the
same loadings except that 50% Mn(NO (A.R)、Cr(NO 9H O (A.R)、BiN 5H O (A.R) and InN
A.R) solutions were adopted as the precursors. The catalysts are labelled as 1%Mn/SnO , 1%Cr/SnO
%Bi/SnO , 1%In/SnO . The chemical components of the samples were verified with ICP in Table 1, which
indicates that for all the samples, the doping metal oxide content is slightly lower than the planned value.
2
3
)
2
3
)
3
2
3
O
9
2
O
3 9
(
2
2
,
1
2
2
2.2 Catalytic activity test and characterization.
The detailed information about the equipment, the running condition, the experimental processes and
parameters are listed in the supporting information file.
3. Results and discussion
6

3.1 XRD, N
2
sorption and Raman analysis
XRD was used to identify the phase structures of all the catalysts. Figure 1(A) depicts that pure SnO
2
has three
strongest diffraction peaks at about 2θ (110) = 27.02 , (101) = 34.34 and (211) = 52.24 which are special for
tetragonal rutile SnO crystalline phase [30]. For the samples doped by different metal oxides, no any phase other
than rutile SnO is detected, which indicates that the secondary metal oxides might disperse finely on the SnO
o
o
o
2
2
2
support surface. Table 1 shows that the mean crystallite sizes of the samples are about 12.7 nm, proving that the
doping amount of oxides do not pose any evident effect on the crystallinity of the support, which is also confirmed
by the constant surface areas of the final catalysts.
The N
relative pressure range of P/P
profiles of the catalysts are similar to that of pure SnO
2
-sorption profiles in Figure S1(A) display that a type Ⅳisotherm with an H3-type hysteresis loop in the
= 08-1.0 is observed for all the samples. Moreover, the pore size distribution
. Whereas, compared with the SnO support, all the
0
2
2
catalysts have smaller pore volumes and diameters, implying that part of the bigger pores are blocked by the
supported metal oxides. This indeed provides evidence to substantiate that the doping metal oxides are finely
dispersed on the SnO
2
support surface and do not affect the texture properties evidently.
12000
24000
(B)
(A)
SnO2 (110) .(101) (211)
A1g
6
33
1
0000
20000
16000
12000
8000
4000
0
Eg
479

B2g
.

777 1% Mn/SnO2
8
6
4
2
000 1%Mn/SnO2
A
1g
Co O4
3
1
% Co/SnO2
% Cr/SnO2
% Bi/SnO2
1
1
1
1
%Co/SnO
000
000
000
0
2
6
90
1
%Cr/SnO
2
1
%Bi/SnO
%In/SnO
2
1
% In/SnO2
2
SnO2
SnO2
20
40
60
80
400
500
600
700
800
900
-
1
2
()
Raman shift (cm )
Figure 1 XRD and Raman results of the catalysts. (A) XRD patterns, (B) Raman spectra.
The structure property of the catalysts was characterized by Raman. Figure 1(B) manifests that pure SnO
2
owns
[40]. All
. Nevertheless, an extra peak at 690 cm is detected
-
1
three peaks at 479, 633 and 777 cm , corresponding to E
g
, A1g, and B2g modes of tetragonal rutile SnO
2
-1
the doped catalysts exhibit similar Raman spectra to pure SnO
2
7

for 1%Co/SnO
symmetry vibration [12]. This further testifies that all the doping metal oxides are mainly dispersed on the SnO
surface as amorphous state except that some Co micro crystallites could be present on 1%Co/SnO surface.
In summary, Raman results are well consistent with the XRD and N -adsorption analysis.
2
, which is associated with the octahedral sites (CoO
6
) of Co
3
O
4
phase and corresponds to A1g
2
3
O
4
2
2
Table 1 Quantified ICP, XRD and N sorption results
2
ICP
Average
crystallite size
(nm)a
BET surface
area
Pore
diameter
(nm)
Pore volume
Sample
Results
3
-1
(
cm g )
2
-1 b
(
wt%)
(m g )
SnO
%In/SnO
%Bi/SnO
%Cr/SnO
2
/
12.7
13.0
12.9
12.5
12.3
12.3
27
29
28
28
27
28
0.2838
0.2358
0.2481
0.2084
0.2414
0.2249
37.06
27.77
31.60
26.30
30.46
28.06
1
2
0.90
0.95
0.92
0.90
0.89
1
1
2
2
1
%Co/SnO
2
1
%Mn/SnO
2
a Calculated by Scherrer’s equation with (1 1 0) reflection of SnO .
2
b Calculated by BET method.
3
.2 Activity evaluation of the catalysts
Toluene deep oxidation activity was studied over the catalysts. It is noted here that under the reaction condition
adopted in this study, toluene is oxidized entirely to CO without forming any other by-products. As depicted in
the Figure 2(A), compared with individual SnO , all the doped catalysts display evidently improved overall
activity, obeying the order of 1%Mn/SnO > 1%Co/SnO > 1%Cr/SnO > 1%Bi/SnO > 1%In/SnO > SnO
exhibits the best activity, over which the complete toluene oxidation is
2
2
2
2
2
2
2
2
.
Among all the catalysts, 1%Mn/SnO
2
o
obtained at 390 C. Apparently, the activity of those catalysts doped by metal oxides having redox property
(
1%Mn/SnO
2
, 1%Co/SnO
2
and 1%Cr/SnO
2
) is obviously better than those doped by metal oxides having acidic
property (1%Bi/SnO
2
and 1%In/SnO
2
).
8

To learn about the inherent activity, the Arrhenius plots of the catalysts have been collected under differential
o
condition in Figure 2(B). Moreover, the reaction rates at 280 C and the apparent activation energies are calculated
for the catalysts and listed in Table 2. As seen directly in Figure 2(C), the reaction rates normalized by the catalyst
weight (R
%Bi/SnO
adding the secondary metal oxides. All the doped catalysts have more or less lower activation energies than pure
SnO , indicating that more surface active sites with higher reactivity could have been formed by adding the
secondary metal oxides.
w
) and catalyst surface area (R
s
) follow also the sequence of 1%Mn/SnO
2
> 1%Co/SnO
2
> 1%Cr/SnO
2
>
1
2
> 1%In/SnO > SnO , testifying strongly that active sites with higher reactivity have been formed by
2
2
2
100
(A)
4
.53999E-5 (B)
1
% Mn/SnO2
80
60
40
20
0
1% Co/SnO
2
1
1
% Cr/SnO2
% Bi/SnO2
1.67017E-5
1% In/SnO2
SnO2
6
2
.14421E-6
.26033E-6
1
1
1
1
1
% Mn/SnO2
% Co/SnO2
% Cr/SnO2
% Bi/SnO2
% In/SnO2
SnO2
250
300
350
400
450
1.55
1.60
1.65
1.70
1.75
1.80
1.85
1.90
-
3
-1
Temperature (C)
1/T (10 K )
25
20
15
10
5
0
25
-
6
-1 -1
(C)
R (10 mmol g s )
w
-
7
-2 -1
R (10 mmol m s )
s
20
15
10
5
0
SnO2
1%In
1%Bi
1%Cr
1%Co
1%Mn
Catalysts
Figure 2 Toluene deep oxidation on the catalysts. (A) Toluene conversion, (B) Arrhenius plots, (C) Toluene
9

conversion rates (R
w
and R ).
s
Table 2 Toluene deep oxidation activity over the catalysts.
R
w
R
s
Ea
-
1
-1
-2 -1
Sample
(mmol g s )
(mmol m s )
-
1
(
kJ mol )
-
6 a
-7 b
(
×10 )
(×10 )
SnO
%In/SnO
%Bi/SnO
%Cr/SnO
2
1.98
7.52
10.4
12.6
17.1
22.1
0.73
2.59
3.71
4.50
6.33
7.89
96.0
89.6
88.5
84.7
81.8
79.8
1
2
1
1
2
2
1
%Co/SnO
2
1
%Mn/SnO
2
o
a Measured at 280 C and normalized by catalyst weight.
o
b measured at 280 C and normalized by catalyst surface area.
3
.3 H -TPR studies on the catalysts
2
H
2
-TPR experiments were conducted to understand the redox properties of catalysts, with the results
o
manifested in Figure 3. All the samples have a main reduction peak at 632 C, being assigned to the reduction of
0
SnO
2
to Sn [37], as also proved by the quantified results in Table S1. However, compared with individual SnO
2
o
and other catalysts, the temperature of this main peak of 1%Co/SnO
2
is 20 C lower, which could be caused by
0
the H
2
spillover effect on the Co species coming from the Co
3
4
O pre-reduction [12,23], as a small amount of
Co
3
O
4
micro crystallites were detected by Raman.
1
0

350
300
250
200
150
100
(
A)
628
(B)
6
12
2
66
1
%Mn/SnO2
3
43
6
38
298
1
%Co/SnO2
632
400
2
70
1
%Cr/SnO2
639
1
%Bi/SnO2
6
32
1
%In/SnO2
50
CVD-SnO2
00
0
2
400
600
800
100 200 300 400 500
Temperature (C)
Figure 3 H -TPR profiles of the catalysts. (A) Complete profiles, (B) enlarged profiles from 200 to 500 °C.
2
It was formerly reported that on SnO
2
surface, substantial amount of active electrophilic oxygen anions can be
o
generated on the surface defects, which is crucial for its reactivity, and can be reduced by H
2
below 400 C.
o
However, this type of oxygen species will be depleted by increasing the calcination temperature above 300 C
[
41-42]. To figure out the impact of the doping metal oxides on this part of active oxygen species, the H -TPR
2
o
profiles of the catalysts are enlarged in Figure 3(B). Since all the samples were calcined at 450 C, as expected,
pure SnO shows no any reduction. For the two samples doped by In and Bi oxides, the deficient oxygen amount
2
is almost negligible either. Interestingly, for the three samples doped by Cr, Co and Mn oxides, very distinct
reduction peaks are detected, testifying that a much larger amount of active electrophilic oxygen species is present
o
on these catalysts. Notably, 1%Co/SnO
2
has a sharp reducing peak at 343 C, which may be assignable to the
0
reducing of micro crystallite Co
3
O
4
to Co , as it was detected by Raman [12,23]. Obviously, with the addition of
a small quantities of these secondary metal oxides, more surface active electrophilic oxygen anions can be
generated and maintained at relatively higher temperatures. Furthermore, the amount of the deficient oxygen in
each sample is nearly in accordance to its toluene oxidation activity, indicating that it could be important for the
11

reaction activity.
3.4 Analyzing the surface active oxygen species by In-situ DRIFTS and EPR
Our former work has demonstrated that on pure SnO
2
samples prepared by different methods, both surface
-
-1
2-
-1
superoxide (O
2
) at around 972 cm and peroxide (O
2
) anions at around 865 cm are detectable with In-situ
DRIFTS technique, which are intimately related to the surface oxygen defects and active electrophilic oxygen
species [31]. Through improving the SnO surface area to generate more surface defects, the amount of these
electrophilic oxygen species is improved, thus enhancing the toluene oxidation activity [30].
To distinguish the surface oxygen sites of the doped SnO samples, they were also studied with In-situ DRIFTS.
Since all the doped samples have similar spectra, only the spectra obtained on 1%Mn/SnO
2
2
2
, the most active
o
sample in this study, are listed and discussed in detail here. First, the sample was pretreated at 400 C in a 30 ml
-1
o
min Ar flow and then cooled down to 300 C. Afterwards, a 10% O
2
/N flow was directed into the reaction cell.
2
-
1
-1
As displayed in Figure 4, three peaks at 775, 834 and 950~1150 cm are observed. The peak at 775 cm
corresponds to Sn-O bond vibration in the plane perpendicular to the SnO
2
c-axis, which is constant with the
-1
2-
increasing of exposure time [43]. The peak at 834 cm is assigned to surface O
2
anions, and the peaks between
-1
-
9
50~1150 cm are attributable to surface O
2
anions species [44-47]. By increasing of exposing time, the
-1
-1
intensities of the peaks at 834 cm and between 950~1150 cm were increased gradually until 15 min. This
2
-
-
testifies that in O
2
atmosphere, more O
2
and O
2
anions can be produced on the surface oxygen vacancies. It is
-
particularly noted here that compared with the spectra of individual SnO
2
in our former study, O
2
anions with
more abundant chemical environments have been generated in these doped catalysts, which is favorable for the
reaction performance [31,48].
1
2

0
0
0
0
0
0
.5
.4
.3
.2
.1
.0
-
2-
O2
O2
2-
O
2
0 min
5 min
0 min
7 min
1
1
5
3
min
min
0 min
Ar
1400
1200
1000
800
600
-
1
Wavenumber (cm )
Figure 4 In-situ DRIFTS spectra of 1%Mn/SnO
2
treated in a 10% O
2
/N
2
flow at 300 °C (0 min, 3 min, 5
min,7 min, 10 min, 15 min, 20 min).
EPR was used to analyze further the surface oxygen species, with the results shown in the Figure 5. Two peaks
at g=1.993 and 1.996 are detected in the spectra of all the samples, which are ascribed to the surface superoxide
O -
Therefore, peroxide O
peaks are different, and follow the sequence of 1%Mn/SnO
%Bi/SnO . This indicates that the addition of Mn, Co and Cr oxides possessing redox property onto SnO
anions [49-50]. It is mentioned here that EPR can only detected those species possessing unpaired electrons.
2
2
-
2
anions cannot be observed with EPR technique. Apparently, the integrated areas of the
> 1%Co/SnO > 1%Cr/SnO > SnO > 1%In/SnO
surface
species, but the addition of In and Bi oxides with acidity can
2
2
2
2
2
>
1
2
2
-
can increase the amount of surface superoxide O
2
inversely decrease its amount.
1
3

1
1
% Mn/SnO2
% Co/SnO2
g=1.996
1000
1
1
1
% Cr/SnO2
% Bi/SnO2
500
% In/SnO2
SnO2
0
-500
-1000
g=1.993
545
550
555
560
565
570
575
580
Magnetic field (mT)
Figure 5 EPR spectra of the catalysts.
3.5 XPS analysis on the catalysts
XPS technique was employed to explore the surface compositions of the catalysts. In Figure S2(A), Sn 3d
spectra show two symmetrical peaks at around 486.4 and 494.8 eV for all the samples, which belong to Sn 3d5/2
4
+
and Sn 3d3/2 of Sn [29,51]. The spectra of the supported metal elements are demonstrated in Figure S2 (B-F).
3
+
4+
3+
4+
The Mn 2p spectrum suggests the co-existence of Mn and Mn with a Mn /Mn atomic ratio of 3.2/1.0 [52-
5
4]. The Co 2p3/2 spectrum shows three peaks at 780.0, 782.4 and 786.0 eV, suggesting the co-existence of Co3+
2
+
3+
2+
and Co with a Co /Co atomic ratio of 2/1 [48]. However, the Cr 2p, Bi 4f and In 3d spectrum indicates the
3
+
3+
3+
presence of only surface Cr , Bi and In cations on the surfaces of the corresponding catalysts [29,55-56]. The
presence of Mn and Co cations with multiple valence states on 1%Mn/SnO
2
and 1%Co/SnO surface might be
2
the reason for them to generate more active surface oxygen sites.
The surface M/(M+Sn) molar percentages of the samples are listed in Table 3. For better comparison, the whole
chemical compositions of the samples measured by ICP in Table 1 are converted also into M/(M+Sn) molar
percentages and presented in Table 3. It is obvious that the surface M/(M+Sn) molar percentages of all the samples
1
4

are higher than the whole molar percentages, testifying again that the doping metal oxides are enriched on the
surfaces, in good accordance to the XRD and Raman results.
6
0000
60000
50000
40000
30000
20000
10000
0
(B)
(A)
5
30.3
530.3
2-
2-
O
O
50000
40000
30000
20000
10000
0
531.6
2-
O2
5
31.7
2-
O2
5
33.0
5
33.0
2
-
O
2
-
O
536
534
532
530
528
526
536
534
532
530
528
526
B.E. (eV)
B.E. (eV)
70000
60000
50000
40000
30000
20000
10000
0
(C)
530.2
(D)
5
4
3
0000
5
30.3
2-
O
2-
O
0000
0000
531.7
2
-
O2
5
31.5
2-
O2
20000
10000
0
533.1
2
-
O
5
32.9
2
-
O
536
534
532
530
528
526
536
534
532
530
528
526
B.E. (eV)
B.E. (eV)
(F)
530.2
(E)
60000
60000
50000
40000
30000
20000
10000
0
530.3
2-
O
2-
O
5
0000
0000
4
30000
20000
10000
0
531.7
5
31.7
2
-
2-
O2
O2
5
33.1
5
33.1
2
-
2
-
O
O
536
534
532
530
528
526
536
534
532
530
528
526
B.E. (eV)
B.E. (eV)
Figure 6 O1s spectra of the catalysts. (A) SnO
2
, (B) 1% Mn/SnO
2
, (C) 1% Co/SnO
2
, (D) 1% Cr/SnO , (E)
2
1
% Bi/SnO
2
, (F) 1% In/SnO .
2
1
5

To understand deeper the surface oxygen properties, the O1s signals have been particularly analyzed for the
catalysts. Figure 6 demonstrates that all the catalysts show an asymmetrical O1s peak, implying the existence of
surface oxygen anions in different chemical environments. Typically, the O1s peak of all the samples is able to
be deconvoluted into three symmetrical peaks at 530.3, 531.7 and 533.1 eV, assignable in sequence to surface
2
-
2-) and superoxide species (O -) [57-58], which matches well the In-situ
lattice oxygen species (O ), peroxide (O
2
2
DRIFTS and EPR results. To distinguish the effect of different doping metal oxides on the surface active oxygen
-
2-
2-
quantity, the (O
2
+ O
catalysts show basically the same (O -
, 1%Mn/SnO catalysts are significantly higher. This testifies strongly that through doping SnO
2
)/O ratios have been calculated for all the samples in Table 3. While 1%In/SnO
2
and
, the ratios of 1%Cr/SnO
with
2-
2-
1
1
%Bi/SnO
2
2
+ O
2
)/O ratios to pure SnO
2
2
,
%Co/SnO
2
2
2
metal oxides possessing redox property, a much larger amount of surface active oxygen sites have been generated,
which may contribute importantly to toluene oxidation. On the contrary, the addition of metal oxides possessing
acidic property onto SnO
%In/SnO and 1%Bi/SnO
catalysts. Whereas, compared with SnO
2
has nearly no contribution to the generation of surface active oxygen sites. Therefore,
show lower toluene oxidation activity than 1%Cr/SnO , 1%Co/SnO , 1%Mn/SnO
support, the doping of In and Bi oxides still improves the toluene
1
2
2
2
2
2
2
oxidation activity, which is mainly due to the increment of the surface acidity, thus facilitating the adsorption and
activation of the toluene molecules. In the following section, this will be discussed in more details with additional
experimental proofs.
Table 3 Quantified XPS results of the catalysts.
Overall
M/(M+Sn)
molar
Surface
M/(M+Sn)
molar
O 1s Binding Energy (eV) and Relative
Amount (%)
-
2-
2-
Sample
(O
2
+O )/O
2
percentage
percentage
-
2-
2-
O
2
O
2
O
a
b
(%)
(%)
SnO
2
-
-
533.0/6.4
533.1/6.3
531.7/21.7 530.3/71.9
531.7/21.7 530.2/72.0
0.39
0.39
1
%In/SnO
2
1.2
5.4
1
6

1
%Bi/SnO
%Cr/SnO
%Co/SnO
2
0.8
2.7
2.2
2.4
2.7
533.1/6.0
533.1/6.4
532.9/6.7
533.0/6.8
531.7/21.8 530.3/72.2
531.7/26.9 530.3/66.7
531.5/28.6 530.2/64.7
531.6/26.2 530.3/67.0
0.38
0.50
0.55
0.49
1
2
12.9
10.7
10.3
1
2
1
%Mn/SnO
2
a Measured by ICP; M=In, Bi, Cr, Co and Mn.
b Measured by XPS; M=In, Bi, Cr, Co and Mn.
3
.6 NH -TPD and Toluene-TPD analysis
3
The surface acid sites on a catalyst are of great importance for its toluene oxidation activity, because the
6
electron rich 휋 bond in a toluene molecule can be easily chelated to them. As a result, the toluene molecules
6
will be activated effectively on the surface acid sites, which will in turn react with the surface active oxygen
anions to form the final products [59-61]. Hence, the surface acidity of the catalysts, which mainly originates
from surface Lewis acid sites, has been characterized by NH -TPD. Figure S3 exhibits that all the samples possess
3
o
a broad NH
3
desorption peak between 50 to 250 C. The quantitative results in Table 4 prove that the amount
obeys the sequence of 1%Mn/SnO
2
>1%Bi/SnO
2
> 1%Cr/SnO
2
> 1%Co/SnO
2
> 1%In/SnO
2
> SnO , which
2
indicates that the surface acidities of all the doped catalysts have been enhanced more or less. Subsequently, this
can influence the adsorption and activation of toluene molecules on the catalysts.
100
1%Mn/SnO2
95
90
85
80
75
70
1
%Bi/SnO2
1
%Cr/SnO2
1
%In/SnO2
1
%Co/SnO2
SnO2
75
80
85
90
95
100
Relative NH desorption amount (a.u.)
3
1
7

Figure 7 The relationship between toluene and NH desorption amount of the catalysts.
3
To verify this, toluene adsorption-desorption experiments have been conducted for the catalysts either. Figure
o
o
S4 exhibits that all the catalysts show two toluene desorption peaks around 150 C (α peak) and above 250 C
(
[
β peak), which are attributed to the weak and medium strength toluene adsorption sites on the catalyst surfaces
30-31]. The addition of doping metal oxides changes evidently the toluene adsorption-desorption behaviors of
the resulted catalysts. Therefore, the toluene desorption amount on the catalysts are also quantified in Table 4.
The total toluene desorption amount increases with the sequence of SnO < 1%In/SnO = 1%Co/SnO
%Cr/SnO < 1%Bi/SnO < 1%Mn/SnO . As manifested intuitively in Figure 7, a catalyst with a bigger quantity
of surface acid sites can generally adsorb a larger quantity of toluene molecules, which is crucial for the activation
of the toluene molecules. In all the samples, 1%Mn/SnO possesses the biggest toluene desorption amount, which
is believed to be another important factor accounting for its best toluene deep oxidation activity.
2
2
2
<
1
2
2
2
2
Table 4 Quantified NH -TPD and Toluene-TPD results of the catalysts.
3
Relative Total
Toluene
Relative Toluene
Desorption
Relative Toluene
Desorption
Relative NH
Desorption
3
Sample
Desorption
Amount
Amount of 훼
peak (a.u.)
Amount of
Amount (a.u.)
훽 peak (a.u.)
(a.u.)
SnO
%In/SnO
%Bi/SnO
2
78
81
95
66
66
78
6
8
72
1
2
74
1
2
11
89
1
8

1
%Cr/SnO
%Co/SnO
%Mn/SnO
2
90
86
47
68
76
39
5
86
73
1
2
1
2
100
24
100
3.7 Toluene Adsorption on the catalyst surfaces measured with In-situ DRIFTS
To gain supplementary information about toluene adsorption on the surfaces of the doped samples, in situ
DRIFTS experiments have been carried out, with the results obtained on 1%Mn/SnO
2
demonstrated in Figure 8.
o
o
The sample was pretreated in a high purity Ar flow at 400 C for 60 min, and then cooled down to 50 C to get
rid of any surface impurity. Afterwards, a N
2
flow containing 1000 ppm toluene was introduced into the in-situ
o
-
cell at 50 C for 60 min. Figure 8(A) manifests that after 5 minutes, four peaks at 1530, 1650, 2904 and 2976 cm
1
-1
were observed simultaneously. The peaks at 1530 and 1650 cm were attributed to the benzene ring in-plane
-1
and out-plane deformation vibration, respectively [62-64]. The 2904 and 2976 cm was ascribed to the stretching
vibration of C-H bonds [63]. This indicates strongly that the toluene molecules start to be adsorbed and activated
on the catalyst surface. In the first 40 min, the intensities of all these peaks increase with exposing time. After
this, the peak intensities remain constant due to surface saturation by toluene molecules.
Figure 8(B) compares the spectra collected on 1%Mn/SnO
2
and pure SnO
2
after saturating both samples in the
are far
is bigger
-TPD and Toluene-
o
1
000 ppm toluene flow for 60 min at 50 C. Apparently, the toluene adsorption bands of 1%Mn/SnO
2
more intensive than those of pure SnO
2
. This proves that the toluene adsorption amount on 1%Mn/SnO
2
than that on SnO
2
, due to presence of more surface acid sites, in good accordance to the NH
3
TPD results.
1
9

0
0
0
0
0
0
0
0
0
.08
.07
.06
.05
.04
.03
.02
.01
.00
0.035
(
A)
(B)
benzene ring
SnO2
0
0
0
0
0
0
.030
.025
.020
.015
.010
.005
benzene ring
1
% Mn/SnO2
1530
650
1
C-H
904
1530
1650
2
976
2
6
0 min
5
0 min
0 min
C-H
2976
4
30 min
0 min
10 min
2904
2
5
0
min
min
0.000
3000
2000
1500
1000
3000
2800
2600 2200 2000 1800 1600 1400 1200 1000
-
1
-1
Wavenumber (cm )
Wavenumber (cm )
Figure 8 In-situ DRIFTS spectra of the catalysts. (A) the 1%Mn/SnO
2
sample treated in a N
2
gas mixture flow
o
containing 1000 ppm toluene at 50 C (0, 5, 10, 20, 30, 40, 50, 60 min); (B) the SnO
2
, 1%Mn/SnO samples
2
o
treated in in a N
2
gas mixture flow containing 1000 ppm toluene at 50 C for 60 min.
To unravel the contributions of surface active oxygen species and acid sites to toluene deep oxidation, the
o
-+O
2-)/O2-
at
toluene conversion rates collected on the catalysts at 280 C are plotted against the corresponding (O
2
2
ratios and toluene desorption quantities in Figure 9. It is clear that a catalyst shows higher activity than SnO
2
least owns a larger quantity of one type of the surface sites. For the samples doped by In and Bi oxides, the limited
activity increase comes mainly from the improved surface acidity, since the amount of surface active oxygen sites
of these two samples has almost no change in comparison with pure SnO . However, for the samples doped by
2
Cr, Co and Mn oxides, the amount of both surface active acid and oxygen sites has been evidently increased, thus
showing significantly improved activity. Therefore, it is rational to conclude that the concerted interaction
between surface active oxygen species and acid sites decides the toluene deep oxidation activity over SnO
2
-based
catalysts.
2
0

-+O
)/O molar ratio and toluene desorption amount.
2-
2-
Figure 9 Toluene oxidation reaction rates (R
w
) versus (O
2
2
3.8 Evaluating the reaction stability, sulfur and water vapor tolerance of the catalysts
o
The reaction stability of 1%Mn/SnO
2
has been tested at 360 C for 50 h. Figure S5 (A) displays that toluene
conversion was stable at around 90% in the 50 h test. To further learn the sulfur and water vapor tolerance of the
o
catalyst, 1%Mn/SnO
ppm SO
After introducing 5% water vapor into the reaction flow, the conversion has about 2% decrease, which can be
fully restored after getting rid of the water vapor. Afterwards, 50 ppm SO was introduced into the reaction flow,
which brought 3% toluene conversion drop, and can be fully regenerated after getting rid of SO . After these tests
with individual poisonous component, both 5% water vapor and 50 ppm SO were directed together into the
reaction flow. At this time, the toluene conversion decreases to 84~85%, but which can still be recovered
completely after removing the poisonous components. Therefore, the H O and SO molecules only compete the
2
sample has been tested at 360 C for 72 h, with the presence of 5% water vapor or/and 50
2
, with the results shown in Figure S5(B). The toluene conversion was first maintained at 90% for 4 hours.
2
2
2
2
2
2
1

surface active sites with the reactants, and have no permanent damage to the catalyst’s surface structure. In
summary, 1%Mn/SnO catalyst possesses not only high activity for toluene deep oxidation, but also superior
stability, water and sulfur deactivation resistance. Therefore, it is a potential catalyst for real industrial application.
2
4
. Conclusions
With the expectation to explore the surface doping effects of metal oxides possessing redox or acidic
properties on SnO
toluene oxidation, SnO
prepared with an impregnation method.
(1) The doping of all the metal oxides can enhance toluene oxidation activity on the prepared catalysts, which
obeys the order of 1%Mn/SnO > 1%Co/SnO > 1%Cr/SnO > 1%Bi/SnO > 1%In/SnO > SnO . Metal
2
surface, and engineering catalysts owning both abundant redox and acid sites for efficient
2
catalysts doped by In, Bi, Cr, Co and Mn oxides on the surfaces have been designed, and
2
2
2
2
2
2
oxides with redox property exhibit more evident improvement effect than those with acidic property on the
activity.
(
2) N
2
-adsorption, XRD and Raman results have indicated that the doping metal oxides are dispersed highly
on the SnO
2
support surface. It has been testified that by modifying SnO
2
surface with Cr, Co and Mn oxides
-
and O
2-) and acid sites
having redox property, the quantities of both the surface active oxygen species (O
2
2
have been significantly improved, thus resulting in remarkable activity increment. In contrast, by modifying
SnO surface with In and Bi oxides having acidic property, only the amount of surface acid sites has been
2
improved, hence having limited improvement effect on the activity.
(3) It is concluded that the synergistic interaction between surface active oxygen species and acid sites controls
the toluene deep oxidation activity over SnO
2
-based catalysts. 1%Mn/SnO catalyst possesses considerable
2
2
2

amount of both types of active sites, thus having the best activity in all the catalysts. Last but not the least,
this catalyst has excellent reaction stability, water vapor and sulfur tolerance, which makes it a potential
catalyst for real industrial application.
CRediT author statement
Yao Guo: Performing the experiments; Processing the data; Writing &Original draft preparation.
Lanling Zeng: Performing part of the experiments; Processing the data.
Xianglan Xu: Helping to supervise the graduate students; Lab management.
Yaqian Liu: Helping partly to supervise the graduate students; Performing part of the experiments.
Yang Liu: Helping partly to supervise the graduate students.
Xionchong Fu: Performing part of the experiments.
Zezhou Gao: Performing part of the experiments.
Zijian Qian: Performing part of the experiments.
Junwei Xu: Helping for catalyst design; Discussing the results.
Xiuzhong Fang: Helping for catalyst preparation; Equipment maintenance.
Xiang Wang: Corresponding author; Supervising; Conceptualization; Revising and editing the original draft.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that
could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge deeply the financial support by the National Natural Science Foundation of China
(
21962009, 21567016, 21666020), the National Key Research and Development Program of China
2016YFC0209302), the Natural Science Foundation of Jiangxi Province (20181ACB20005, 20171BAB213013,
(
20181BCD40004, 20181BAB203017).
2
3

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